Hydrolysis of silicon tetrafluoride

ABSTRACT

A mixture comprising oxygen, a hydrocarbon fuel and silicon tetrafluoride is burned, the amount of oxygen in the mixture being insufficient for complete combustion of the fuel. A substantial portion of the silicon tetrafluoride is hydrolyzed within the resulting incomplete-combustion hydrocarbon flame, oxygen being added thereafter for combustion of the remaining fuel therein. Probably because of free radical initiation in the incomplete-combustion flame, the conversion efficiency of silicon tetrafluoride to silica can be maintained at high levels while temperatures remain below the fusion-sintering point of the silica, i.e., not in excess of about 1,600* F.

United States Patent [151 3,661,5 1 9 Driscoll May 9, 1972 54]HYDROLYSIS 0F SILICON 3,130,008 4/1964 Stokes et al...... .....23/l82 vx TETRAFLUORIDE 3,203,759 8/1965 Flemmert... ..23/l82 V 3,233,969 2/1966Heller ..23/1 82 [72] lnventor: Richard E. Driscoll, Monroe, La.

[73] Assignee: Cities Service Company, New York, NY. Primary mmi nerEdward stem Attorne v.l. Richard Geaman [22] Filed: July 1, 1970 [5 7]ABSTRACT A mixture comprising oxygen, a hydrocarbon fuel and silicontetrafluoride is burned, the amount of oxygen in the mixture beinginsufficient for complete combustion of the fuel. A substantial portionof the silicon tetrafluoride is hydrolyzed within the resultingincomplete-combustion hydrocarbon flame, oxygen being added thereafterfor combustion of the remaining fuel therein. Probably because of freeradical initiation in the incomplete-combustion flame, the conversionefficiency of silicon tetrafluoride to silica can be maintained at highlevels while temperatures remain below the fusion-sintering point of thesilica, i.e., not in excess of about 1,600 F.

9 Claims, 2 Drawing Figures 5i F 4 H2 0 A/R-I- FUEL GAS SECONDARY AIR I5I I COOLING AIR [52] U.S.Cl. ..23/l82 V, 23/1 B, 23/153, 23/182 P [51]Int. Cl. ..C01b 33/18, COlb 33/14 [58] Field of Search ..23/1 82 V, 182,202 V, l R, 23/1 B, 153

[56] References Cited UNITED STATES PATENTS 2.8l9,l5l l/l958 Flemmert..23/l82 V 3,086,851 4/1963 Wagner 9 [ll I P'A'TE'N'TEDMAY 9 19723,661,519

SHEET 1 BF 2 $iF4+ H20 5 AIR+ FUEL GAS SECONDARY AIR F I I c001. ING AIRFig. l

SILICA AEROSOL INVENTOR.

RZCHAfQL D RISCOLL I L Z- A h BY ifromvsv PATENTEDMY 9 I97? SHEET 2 BF 2HMMM 0.6 k0 29mmm 2ou n.

INVENTOR. RICHARD E. DRISCGU.

ATTORNEY HYDROLYSIS OF SILICON TETRAFLUORIDE BACKGROUND OF THE INVENTIONThe present invention pertains to manufacture of amorphous, pigmentarysilicon dioxide which is produced by hydrolysis of silicon tetrafluorideat elevated temperatures, e.g., in excess of about l,l F. The mostwidely utilized technique for achieving the hydrolysis has been flameprocesses wherein a fluid fuel is burned with a free-oxygen containinggas in the presence of the silicon tetrafluoride, water for the reactionbeing supplied by combustion of the fuel or from an external source. Theresulting silicas generally have a particle size within the range ofabout -50 millimicrons and a surface area within the range of about75-200 square meters per gram. After formation, the silica is separatedby filtration from gaseous products of the reaction, e.g., hydrogenfluoride, water vapor and CO and is recovered as a dry, low densitypowder. Among other uses, such silicas can be employed as reinforcingagents for silicone rubber, as thickeners for resins and greases, and asanticaking agents.

US. Pat. No. 2,631,083 discloses that the hydrolysis of silicontetrafluoride will proceed 80 percent to completion at 1,500 F. whiletemperatures in excess of l,800 F. are required for substantiallycomplete hydrolysis, e.g., in excess of 90 percent. However, theaforesaid disclosure fails to indicate that prohibitively long reactiontimes are required to achieve conversion efficiencies over 50 percentwhen the reaction temperature is below about 1,600 F. As a consequencethe art has developed around flame processes wherein the flame has ahigh intensity; or, more generally, a temperature of at least 1,800" P.and commonly in excess of 2,000 F. Thus, operating techniques have beendeveloped which effect substantially complete combustion of the fuel inthe flame in the shortest possible length of time since this assuresrapid and highly efficient conversion of the silicon tetrafluoride tosilica.

Flame processes for manufacturing silica from silicon tetrafluoride havenontheless been plagued by fusion or sintering of the micro-particleswhich occurs at temperatures of about l,600 F. and higher and which issometimes observed at temperatures as low as about l,450 F. Temperaturesabove the fusion-sintering point can result in considerable lowering ofthe surface area of the silica unless the particles are quenched rapidlyenough. A more serious problem has been the formation of accretions onthe walls of the hydrolysis chamber, i.e., the hot silica particlesstick to the wall and to each other and quickly form deposits whichalter mixing and flow patterns of the reactants and which willeventually plug up the chamber unless they are cleaned out fairlyfrequently. Not only does the clean-out procedure require shutdown ofthe process, but a considerable proportion of the silica product is lostto these deposits, which are worthless, since they cannot be reconvertedto useful pigmentary silica by pulvarization or grinding.

The need has persisted, therefore, for a flame process wherein theconversion of silicon tetrafluoride to silica can be carried out at highefficiency levels while employing hydrolysis temperatures which arebelow the sintering-fusion point of the silica being produced.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide a flame process wherein high yields of silica are achieved bythe hydrolysis of silicon tetrafluoride at temperatures which avoidsintering and fusion of the silica. Other objects and advantages of theinvention will become apparent from the following description thereofand the appended claims.

In accordance with the present invention a mixture is burned whichcomprises oxygen, a hydrocarbon'fuel gas and silicon tetrafluoride, theamount of oxygen in said mixture being insufficient for substantiallycomplete combustion of the fuel. The silicon tetrafluoride issubstantially hydrolyzed within the incomplete-combustion flame whichresults from burning of the mixture. Thereafter, more oxygen is added tothe flame for burning of the combustible fuel components which remaintherein, e.g., carbon monoxide, hydrogen, aldehydes, etc.

In addition to oxygen, fuel, and silicon tetrafluoride, the hydrolysisreaction mixture can comprise extraneously supplied water vapor andinert gases such as nitrogen and hydrogen fluoride. In any case thecomponents of the mixture can be proportioned so that upon substantiallycomplete ignition of the fuel, flame temperatures do not exceed aboutl,600 F.

An essential feature of the invention is substantial hydrolysis of thesilicon tetrafluoride in a region of the flame wherein combustion of thehydrocarbon fuel is substantially incomplete, since this step isnecessary to the achievement of high yields of silica at a relativelylow hydrolysis temperature. Although not wishing to be bound by theory,it is felt that free radicals-especially hydroxyl-which are liberated inthe flame are highly reactive with the silicon tetrafluoride, thusachieving what can only be accomplished with water at the highertemperatures required for sufficient dissociation thereof. By means ofthe present invention the hydrolysis of silicon tetrafluoride can,therefore, be carried out at temperatures within the range of aboutl,200-l ,600 F. while achieving conversion efficiencies of percent andhigher. To advantage, the hydrolysis temperatures can be maintainedwithin the range of about l,200 F. to about l,400 F. to provide anincreased margin of safety against fusion and sintering of the silica.

Any suitable fluid hydrocarbon fuel can be employed for forming thehydrolysis reaction mixture, but normally gaseous hydrocarbons can beemployed to advantage, i.e., methane, ethane, propane, butane ormixtures thereof. Since hydrogen can also be employed in the practice ofthe invention, it will be understood that the term hydrocarbon fuel gas"as used herein is intended to mean hydrogen as well as mixtures thereofwith other hydrocarbon fuels. Oxygen may be supplied to the hydrolysisreaction mixture as substantially pure oxygen, air, air enriched withoxygen, or any other suitable free oxygen-containing gas.

As previously indicated the flame in which the hydrolysis takes place ischaracterized by a first region in which the proportion of fuel andoxygen is such that combustion of the fuel is substantially incomplete,and a second region wherein further mixing and combustion with oxygenoccurs. The proportion of oxygen to fuel which can exist in the firstregion for incomplete combustion of the fuel is subject to considerablevariation, but the fuel should be in excess to the extentthat thestoichiometric fraction, 4), is at least about 1.3 where:

V0lume percent of fuel gas in actual mixture (V.F. G.) stoichiometricfuel gas, volume percent (S.F.G.)

To particular advantage of the stoichiometric fraction, (1:, can bewithin the range of about 1.5 to about 1.8.

After partial combustion of the fuel and substantial hydrolysis of thesilicon tetrafluoride within the incomplete combustion flame, oxygen isadded to the flame in an amount which is sufficient for substantiallycomplete ignition of unburned fuel constituents which remain therein.Some excess oxygen can be added where it is preferable and practical todo so. Accordingly, the total amounts of fuel and oxygen which are fedto the entire flame will generally be in a proportion such that is 1.0or less, i.e., stoichiometric or lean in fuel.

While it is desirable to maintain the hydrolysis temperatures belowabout l,600 F., the amount of fuel which is burned for hydrolysis of anygiven amount of silicon tetrafluoride is nonetheless subject toconsiderable variation, since more or less heat may be required forheating up variable amounts of water vapor and inert gases which can beincorporated into the hydrolysis reaction mixture, the actual amounts ofthese constituents being dependent, among other things, upon the mannerin which the silicon tetrafluoride is generated and the amount ofnitrogen, etc., in mixture with the oxygen stream.

Air, for instance, can be conveniently employed for combustion of thehydrocarbon fuel, and the silicon tetrafluoride can be in mixture watervapor prior to incorporation into the hydrolysis mixture.

In the present invention, the concentration of silicon tetrafluoride inthe reaction mixture can be varied for the purpose of regulating thesurface area of the silica being produced, this being the subject ofanother invention which is described in my copending patent applicationSer. No. 51,632 filed July 1, 1970. By means of the process disclosedtherein, the concentration of silicon tetrafluoride in the hydrolysismixture is established at a selected value which corresponds in directproportion to the surface area desired of the silica product, i.e., thesurface area of the silica is increased by raising the concentration ofsilicon tetrafluoride in the hydrolysis mixture, and vice versa.Accordingly, the constituents which are formed into the reaction mixturecan be proportioned to provide a preselected mass ratio of flamereactants and diluents to silicon tetrafluoride which is within therange of about to about 1 10, thereby permitting the formation ofsilicas having a particle size of about 5 to about millimicrons and aspecifically desired surface area (B.E.T.) within the range of about 200to about 500 m lgm. Flame reactants are the hydrocarbon fuel and oxygen,while diluents include water vapor, hydrogen fluoride and nitrogen, thelatter being introduceable as air whereby oxygen is also supplied forcombustion of the fuel. To advantage, the silicon tetrafluoride can bepassed into the hydrolysis zone while mixed with a preselected amount ofa diluent vapor, thus providing a means for regulating the concentrationof silicon tetrafluoride in the mixture while maintaining a constantfeed rate of the reactants thereto. Suitable diluents for this purposeinclude water vapor, hydrogen fluoride, inert gases, and mixturesthereof. When using air to supply oxygen to the flame, the content ofsilicon tetrafluoride and diluent vapor in the mixture can beproportioned to provide a specific mol fraction of silicon tetrafluoridetherein which is within the range of about 0.02 to about 0.14.Accordingly, a specific concentration of silicon tetrafluoride in thereaction mixture can be established whereby the silica product has asurface area that is specifically desired. i

Hydrolysis of the silicon I the invention results in formation of anaerosol of the silica suspended in flame gases and hydrogen fluoride.The silica is then separated and recovered from these gases by means ofknown techniques, e.g., after cooling, the aerosol is passed into a bagfilter wherein thesilica is separated at a temperature of about 450-500F.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a horizontal view, insection, of a hydrolysis reactor which can be used in the practice ofthe invention.

FIG. 2 is a graphical presentation of silica yield versus (1: of theincomplete-combustion flame when hydrolyzing silicon tetrafluorideduring one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS With reference to FIG. 1, ahydrolysis chamber 1 is enclosed by a metal wall 2 having tapered inletand outlet ends represented at 3 and 4, respectively. A burner forforming the incomplete combustion flame is generally represented at 5.The burner comprises a barrel 6 with a somewhat enlarged discharge endwhich contains a refractory flame-stabilizer ring 7. A mixture of airand hydrocarbon fuel gas is fed to the burner barrel 6 through conduit 8while a mixture of silicon tetrafluoride and water vapor is introducedaxially through conduit 9. The resultant mixture of air, fuel, silicontetrafluoride and water vapor is ignited at the discharge end of theburner and burns as a flame which is generally represented at 10.Secondary air, for combustion of the fuel components which reside in theflame after burning of the fuel-rich hydrolysis mixture, is introducedtangentially into a burner shroud tetrafluoride in accordance'with 11through conduit 12. A silica aerosol thus forms in chamber 1 byhydrolysis of the silicon tetrafluoride. The aerosol is removed from thechamber through conduit 13 and is conveyed therein to cooling andcollection. units for recovery of the silica. The metal wall 2 of thehydrolysis chamber is provided with a cooling jacket 14 into whichcooling air is introduced through conduit 15, and is removed therefromthrough conduit 16.

In operation, a fuel-rich mixture of air and fuel gas, i.e., whereby 4ais at least about 1.3, is introduced into the burner through conduit 8at a constant rate. At the same time, a mixture of silicon tetrafluorideand water vapor is fed to the burner through conduit 9. Advantageously,the silicon tetrafluoride and water can be proportioned in the mixtureto provide a selected mol fraction of silicon tetrafluoride which iswithin the range of about 0.02 to 0.15 for control of the surface areaof the silica. A fuel supply rate is selected which will provide atemperature of not in excess of about 1,600" F. in the hydrolysis flame.

The hydrolysis reaction mixture, being rich in fuel, burns as arelatively cool elongated flame along the center-line of the hydrolysischamber. Since, however, the secondary air is introduced in a manner sothat it surrounds the flame rather than being quickly and forcefullymixed'with it, substantially complete hydrolysis of the silicontetrafluoride occurs in a region of the flame wherein combustion of thefuel is substantially incomplete. As a consequence, unburned fuel issubjected within this region to thermal cracking and/or only partialreaction with oxygen whereby free radicals are formed which survive longenough for reaction in a manner which does not exist in a hot, fastflame, thus effecting high yields of silica at unusually lowtemperatures. The temperature within the region of the flame whereinincomplete combustion and substantial hydrolysis occurs may, forinstance, be within the range of only about l,200-1,400 F., whereas thetemperature may increase to l,600 F. in the region wherein combustion ofthe fuel is completed by the addition of oxygen.

Mixtures of silicon tetrafluoride and water vapor for incorporation into,the hydrolysis reaction mixture may be produced in any suitable manner.Such mixtures are stable within the range of just above dew point toabout 600 F. The mixture may be produced, for instance, by thevaporization of hydrofluosilicic acid, but more advantageously theprocesses as described in US. Pat. Nos. 3,233,969 or 3,273,963 can beemployed. It will be understood, nonetheless, that substantially drysilicon tetrafluoride can also be utilized in the invention, and can beproduced, for instance, by reacting hydrogen fluoride with silica in apolyhydric alcohol bath. Such a process is described in US. Pat. No.2,891,872.

EXAMPLE Using a hydrolysis reactor substantially as shown in FIG. 1,eight experiments were run at fixed conditions except that the ratio ofair to fuel gas introduced into the burner through conduit 8 was variedto provide values of 1: within the range of about 0.95 to about 1.5 inthe incomplete combustion flame. To produce the mixture of silicontetrafluoride that was fed to the burner through conduit 9, 30 percenthydrofluoric acid was vaporized at the rate of 30 lbs/hr. and theresultant vapor was passed over a bed of silica pebbles maintained at250 F. to provide silicon tetrafluoride in mixture with water vapor anda little HF, i.e., conversion of the HP to silicon tetrafluoride waspercent efficient. The composition of the resulting feed stream, whichentered the burner through conduit 9, is shown in Table I.

Natural gas, in mixture with varied amounts of air, was fed to theburner at the rate of 250 SCFH through conduit 8, this mixture beingpreheated to 350 F. Secondary air, also preheated to 350 F was fed tothe hydrolysis chamber through conduit 12 at rates to establish anoverall ratio of air to fuel of 12/], the theoretical stoichiometricratio being about 9.6/1. In each experiment the total air input to thehydrolysis zone was maintained at 3,000 SCFH.

The composition of the air-fuel mixtures introduced into the burnerthrough conduit 8 is shown in Table II. In Experiments 3-8 thesemixtures provided an incomplete combustion flame in which substantialhydrolysis of the silicon tetrafluoride occured. Yield values for eachexperiment are also shown.

A graphical representation of the yield value of silica versus d) forthese experiments is shown in FIG. 2. It is apparent therefrom thatyield values did not exceed about 50 percent when dz for the oxygen-fuelgas mixture of the incompletecombustion flame was lower than 1.0. Foryield values in excess of 80 percent, 4) exceeded about 1.3.

The following example is given for calculating (16, using astoichiometric mixture of methane in air for the purpose ofillustration, 9.56 volume units of air being required for combustion ofone volume unit of methane.

Volume percent fuel gas in actual mixture (V.F.G.)

Stoichiometric fuel gas, volume percent (S.F.G.)

As can be determined from this example, fuel mixtures wherein dz is lessthan 1 are lean in fuel whereas they are rich in fuel when d: is inexcess of l.

During each of the aforegoing experiments, the temperature of thehydrolysis flame did not exceed about 1,600 F. The resulting silicaaerosols were passed into a bag filter wherein the silica was separatedfrom the gases at about 450 F. Each of the nilicun was examined andfound to have a particle size of about 9 millimicrons and 5 surface areaof 300 square meters per gram. No fused accretions of the silica werefound in the hydrolysis chamber after the experiments were completed.

This invention has been described with reference to particularapparatus, conditions, proportions and the like, but it will beunderstood that various changes and modifications can be made which arewithin the spirit and scope of the invention as is defined in thefollowing claims.

Therefore, whatis claimed is:

l. A process for producing pigmentary silica having a particle sizewithin the range of about 5 to about 15 millimicrons and a surface areawithin the range of about 200 m lgm. to about 500 mlgm. by hydrolysis ofsilicon tetrafluoride in hot flame gases produced by combustion of ahydrocarbon fuel with oxygen which comprises:

a. burning a mixture comprising oxygen, a hydrocarbon fuel gas andsilicon tetrafluoride, wherein said fuel is in excess with respect tosaid oxygen to the extent that the stoichiometric fraction, 5, is atleast about 1.3 where:

b. substantially hydrolyzing said silicon tetrafluoride within theincomplete-combustion flame which results from the burning of saidmixture,

c. thereafter adding oxygen to said flame and substantially completelyburning combustible fuel constituents which remain therein, andwhereupon substantially complete ignition of said fuel the flametemperature does not exceed about 1,600 F., and wherein said silicontetrafluoride is hydrolyzed at a temperature within the range of aboutl,200 F. to about l,600 F.,

d. thereafter recovering silica from the resulting aerosol.

2. The process of claim 1 wherein the flame temperature is within therange of about l,200-l ,400 F.

3. The process of claim 1 wherein the fuel gas is selected from thegroup consisting of methane, ethane, propane, butane, hydrogen andmixtures thereof.

4. The process of claim 1 wherein the oxygen that is supplied to themixture that is burned is furnished thereto as a gas selected from thegroup consisting of substantially pure oxygen, air, and air enrichedwith oxygen.

5. The process of claim 1 wherein the mass ratio of flame reactants anddiluents to silicon tetrafluoride which are formed into the hydrolysismixture is within the range of about 10 to about 110.

6. The process of claim 1 wherein oxygen is supplied to the hydrolysismixture in the form of air while silicon tetrafluoride is suppliedthereto in mixture with a diluent vapor.

7. The process of claim 6 wherein the diluent vapor comprises watervapor.

8. The process of claim 6 wherein the mo] fraction of silicontetrafluoride in the diluted mixture is within the range of about 0.02to about 0.14.

9. The process of claim 1 wherein the stoichiometric fraction, 4:, iswithin the range of about 1.5 to about 1.8.

2. The process of claim 1 wherein the flame temperature is within therange of about 1,200*-1,400* F.
 3. The process of claim 1 wherein thefuel gas is selected from the group consisting of methane, ethane,propane, butane, hydrogen and mixtures thereof.
 4. The process of claim1 wherein the oxygen that is supplied to the mixture that is burned isfurnished thereto as a gas selected from the group consisting ofsubstantially pure oxygen, air, and air enriched with oxygen.
 5. Theprocess of claim 1 wherein the mass ratio of flame reactants anddiluents to silicon tetrafluoride which are formed into the hydrolysismixture is within the range of about 10 to about
 110. 6. The process ofclaim 1 wherein oxygen is supplied to the hydrolysis mixture in the formof air while silicon tetrafluoride is supplied thereto in mixture with adiluent vapor.
 7. The process of claim 6 wherein the diluent vaporcomprises water vapor.
 8. The process of claim 6 wherein the molfraction of silicon tetrafluoride in the diluted mixture is within therange of about 0.02 to about 0.14.
 9. The process of claim 1 wherein thestoichiometric fraction, phi , is within the range of about 1.5 to about1.8.